Technical Field
The present disclosure relates to a driving circuit that drives voltage-controlled switches.
Description of the Related Art
Such driving circuits as disclosed in JP-A-2013-240210, for example, drive switches are configured of an upper arm portion and a lower arm portion which are connected in series. The switches are IGBTs, for example. Each of the upper arm portion and the lower arm portion is configured of one switch.
As the drive circuits, some of the drive circuits drive a plurality of switches mutually connected in parallel. The parallel connected circuit composed of the switches constitutes the respective upper arm portion and lower arm portion.
In the drive circuit, capacitances are formed at a gate terminal and between the output terminals. Hence, loop paths are formed having electrical conduction paths in the drive circuit which is connected to the gate, the output terminal, and the gate-output terminal.
On the other hand, a recovery current flows through a freewheel diode connected in reverse parallel to the drive object switch. When the recovery current stops flowing, a surge voltage occurs in a conduction path of the recovery current. In this case, among the plurality of switches, the electrical potential at the output terminal of the drive object switch corresponding to the flywheel diode as a source of the surge voltage becomes relatively low compared to voltages of the output terminals in the other drive object switches. Such a voltage difference occurs because the output terminals of the plurality of drive object switches are mutually connected.
When the voltage difference occurs, LC resonance occurs between an inductance component of the above-mentioned loop path and a capacitance component of gate-output terminal. When this LC resonance occurs, the applied voltage of respective terminals such as gates and output terminals may exceed the ratings thereof, and may cause malfunctions of the switches.
The completion timing of the recovery current may vary at at least two flywheel diodes among the respective flywheel diodes, because of individual differences of the flywheel diodes. In this case, because the recovery current of the flywheel diodes subsequently stops flowing, voltage differences between output terminals of the drive object switches significantly vary. Large variation of the voltage differences is likely to cause the above-mentioned malfunctions of the drive object switches.
In order to deal with these problems, it is considered that switching rate of the switches may be lowered when the switches turn ON from the OFF state. However, in this case, power loss increases when the drive object switches turn ON.